17 research outputs found
Vibrational Properties of Nanoscale Materials: From Nanoparticles to Nanocrystalline Materials
The vibrational density of states (VDOS) of nanoclusters and nanocrystalline
materials are derived from molecular-dynamics simulations using empirical
tight-binding potentials. The results show that the VDOS inside nanoclusters
can be understood as that of the corresponding bulk system compressed by the
capillary pressure. At the surface of the nanoparticles the VDOS exhibits a
strong enhancement at low energies and shows structures similar to that found
near flat crystalline surfaces. For the nanocrystalline materials an increased
VDOS is found at high and low phonon energies, in agreement with experimental
findings. The individual VDOS contributions from the grain centers, grain
boundaries, and internal surfaces show that, in the nanocrystalline materials,
the VDOS enhancements are mainly caused by the grain-boundary contributions and
that surface atoms play only a minor role. Although capillary pressures are
also present inside the grains of nanocrystalline materials, their effect on
the VDOS is different than in the cluster case which is probably due to the
inter-grain coupling of the modes via the grain-boundaries.Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev.
Frações de fósforo após extrações sucessivas com resina e incubação, em Latossolos e Luvissolos do semi-árido de Pernambuco
Incidence of Phytophthora infestans (Mont.) de Bary on potato and tomato in Maine, 2006–2010
Late blight, caused by Phytophthora infestans (Mont.) de Bary, is a devastating disease which is found worldwide. In Maine,
United States (US), we recorded late blight on potato and tomato during the 2006–2009 cropping seasons. From 2006 to 2008, over 90%
of the diseased samples were collected in potato fields from northern and central Aroostook County in Northern Maine, US. Then, in
2009, an unprecedented influx of inoculum on infected tomato transplants shipped to retail garden centers throughout the Northeast
US significantly changed the late blight infection patterns. In 2009, 43% of diseased samples obtained were from tomato, and 57% from
potato, and disease was found to occur all over the state. Moran’s index and spatial autocorrelation analysis of disease occurrence,
geographical locations, host factors, and infection levels from previous years, were not statistically significant (p > 0.05). Therefore,
random distributions of late blight incidences were recorded across locations and years. Nearest neighbor analysis revealed that mean
spatial distances for late blight occurrence ranged from 1.51 to 71.4 km from 2006 to 2008, and 7.4 to 126.5 km in 2009. The frequency
and locations of late blight outbreaks in 2009 were substantially greater than in 2006, 2007, and 2008, as affected by the influx of inoculum
and movement of infected tomato seedlings as well as conducive environmental conditions. All were contributing factors for late
blight occurrence in Maine. In 2010, few disease samples were collected, indicating that the influx of inoculum in 2009 did not persist
to cause widespread disease in 2010. The reduction of inocula sources, fungicide protection of susceptible hosts, and the removal
and destruction of infected tomato seedlings and potato cull piles or volunteer plants, can greatly reduce late blight occurrences and
improve potato production. These actions should be considered as an integral part of late blight management programmes in regions
where late blight commonly occurs
Survival potential of Phytophthora infestans sporangia in relation to environmental factors and late blight occurrence
Potato is an important crop globally and late blight (Phytophthora infestans) often results in severe crop loss. The cost for late
blight control can be in excess of $210 million in the United States. We utilised a non-parametric density distribution analysis of local
temperature (T) and relative humidity (RH), from 2005 to 2009, to assess and validate sporangia survival potential using survival
model and late blight risks during the potato cropping season at Presque Isle, in the northern part of the state of Maine, USA. Modelbased
analyses showed that ambient temperatures of 3−30°C and RH values of 45−100% were conducive for sporangia survival. Disease
outbreaks and risk periods coincided with a high sporangia survival probability (15−35%). Due to the omission of solar radiation
(SR) in the computation of survival potential in previous research, we applied a Cox proportional model to estimate the probability of
sporangia survival [i.e. hazard at a specific time H(t)] as a function of baseline hazard (H0) and the influencing parameters. The model
is: H(t) = H0(t) × exp(0.067ET + 0.138T + 0.083RH + 0.001SR) where ET is exposure time. The survival model indicated that RH (β = 0.083)
and T (β = 0.138) were significant (p < 0.05) factors in sporangia survival in comparison to SR (β = 0.001). The hazard ratio, indicative of
sporangia survival risk, varied with the predictors. For the unit increase of T, sporangia survival hazard increased by 1.148 times. The
Cox model and sporangia hazard probabilities can be used for short-term disease forecasts based on the risk period most conducive
for pathogen survival and targeted fungicide applications for optimum late blight management
Nitrogen mineralization from broiler litter applied to southeastern Coastal Plain soils
A field study was conducted to determine nitrogen (N) mineralization from broiler litter (EL) in two Coastal Plain soils of differing texture, sandy (Tifton loamy sand) or clayey (Greenville sandy clay loam). These soils represented the broad range in surface textures commonly found in soils used for agricultural production in the southeastern Coastal Plain. Published protocols used for the study were designed by the ARS mineralization team. In addition to measuring ammonium (NH4-N) and nitrate (NO3-N) in the soil as a measure of N mineralization, both total C and total N were measured to determine the impact of a single BL amendment on C sequestration and N accumulation. Amounts of N in the soil from BL mineralization over 70 days were identical for both soils, 46.4 mg N kg-1 soil (0.046%), but differences occurred in timing of the mineralization processes. In the sandy Tifton soil, depletion of NH4-N and nitrification of the NH4-N to NO3-N occurred simultaneously. The NH4-N from the BL was depleted in 21 days while peak NO3-N concentrations in the soil were found at 28 days. In the clayey Greenville soil, NH,-N concentrations from BL mineralization increased for 21 days and then decreased until reaching background levels by 70 days. Nitrate concentrations never did increase in the BL amended Greenville soil, indicating both that the nitrification rate was much slower than the ammonification race, and most likely, that what NO-N was produced was lost from the soil by denitrification under wet conditions. The combination of soil textural and microclimate differences along with greater protection of the BL residues in the clayey soil than in the sandy soil are believed responsible for the observed N mineralization differences between the two soils. Previous research has shown that N mineralization rate is positively correlated with sand content and negatively correlated with clay content of soils, and the results of this study concurred with those findings. Measurements of total C and total N in both Coastal Plain soils showed that overall increases were small with a single BL amendment, and it was concluded that long-term studies are needed to investigate C sequestration and N accumulation. It was concluded from the study that there is a high probability that BL mineralization rates will be significantly slower on the more clayey Coastal Plain soils than on very sandy ones, and that farm managers should take these rates into consideration when planning timing and amounts of BL applications